An OTN (Optical Transport Network) is high-speed transmission technology based on SONET/SDH (Synchronous Optical Network/Synchronous Digital Hierarchy) and designed so as to be capable of accommodating SONET/SDH and Ethernet (registered trademark) based services. The frame structure is specified by ITU-T (International Telecommunication Union Telecommunication Standardization Sector) G. 709 and the overhead has a shape similar to that of SONET/SDN. The digital layer of the OTN is formed basically by three hierarchies and accommodates a client signal in an ODU (Optical channel Data Unit) via an OPU (Optical channel Payload Unit) and maps the signal to a wavelength via an OTU (Optical channel Transport Unit).
For a network, the protection technique against failures is indispensable in terms of management. In the OTU also, the linear protection is specified in ITU-T G. 873.1.
FIG. 14 explains the linear protection. In FIG. 14, nodes 101 and 102 are illustrated. The nodes 101 and 102 have protection switch units 101a and 102a, respectively. It is assumed that the protection switch unit 101a is a bridge and the protection switch unit 102a is a selector.
Between nodes, a unique signal number is attached by, for example, an operator. For example, in the case of FIG. 14, between the nodes 101 and 102, a signal number Sig1 is attached.
In FIG. 14, between the nodes 101 and 102, paths of an active ODU and a spare ODU are established and when a failure occurs, the protection switch units 101a and 102a switch the paths for a client signal at the ODU layer level.
For example, when a failure occurs in the active ODU, the protection switch unit 101a of the node 101 transmits a switch request to the signal number Sig1 by an APS (Automatic Protection Switching) message of the spare ODU in order to request to switch the paths. Upon receipt of the switch request to the signal number Sig1 by the APS message of the spare ODU, the protection switch unit 102a of the node 102 returns its response to the protection switch unit 101a by an APS message of the spare ODU.
Due to this, the protection switch unit 101a may send out a client signal to the spare ODU and the protection switch unit 102a may receive the client signal from the spare ODU. When the recognitions at both two end nodes agree with each other, it is possible to perform switching due to a failure even if the signal number in the linear protection is not understood by a relay node.
Conventionally, in addition to the already existing services (DS1, DS3, STS-1, etc.), there is proposed an add and drop multiplexer capable of dealing with a fixed-length cell, such as an ATM (Asynchronous Transfer Mode) cell (for example, see Japanese Laid-open Patent Publication No. 09-93254). Further, an information communication system in which a misconnect or traffic drop does not occur at the time of setting of a communication path, a monitoring controller used in this information communication system, a transmission equipment, and a path setting method are proposed (for example, see Japanese Laid-open Patent Publication No. 2000-156694).
The OTN is also supposed to be operated in a ring topology and ITU-T G. 873.2 has begun reviewing of the ODU ring protection. When the same APS message as that of the linear protection is used, there used to be such a problem that a misconnect of a client signal occurs when a duplicated signal number is attached between nodes in a ring topology and path switching is performed due to the occurrence of a failure.
FIG. 15 explains a misconnect in a ring topology. In FIG. 15, nodes 111 to 116 are illustrated. The solid line double-pointed arrow illustrated in FIG. 15 indicates an active ODU and the dotted line double-pointed arrow indicates a spare ODU. It is assumed that between respective nodes 111 to 116, signal numbers Sig1 to Sig6 are attached as illustrated in FIG. 15.
Here, it is assumed that a failure has occurred in the active ODU and the spare ODU between the nodes 111 and 112. In this case, the node 111 transmits a switch request to the signal number Sig1 in which the failure has occurred by the APS message of the spare ODU to the node 116.
Because the signal number of the traffic the node 116 should terminate (receive) is ‘Sig5’, the node 116 establishes a crossconnect so as to let the switch request to the signal number Sig1 received at the spare ODU pass through and pass through the spare ODU. The nodes 115, 114, and 113 also let the switch request pass through similarly.
The node 112 performs switching so as to receive the signal number Sig1 from the spare ODU because the signal number of the traffic the node 112 should terminate is ‘Sig1’. Upon receipt of the APS message of the switch request to the signal number Sig1, the node 112 transmits the response APS message at the spare ODU via the nodes 113 to 116. Due to this, the node 111 may send out the client signal to the spare ODU and the node 112 may receive the client signal from the spare ODU.
The above is a case where a signal number is allocated uniquely by an operator. In contrast to this, for example, when an operator attaches the signal number Sig1 (enclosed by brackets in FIG. 15) between the nodes 115 and 116, the node 116 terminates the signal number Sig1 received from the node 111 at the spare ODU. Consequently, the client signal to be transmitted from the node 111 to the node 112 at the spare ODU is terminated at the node 116, resulting in a misconnect (arrow A101 in FIG. 15). Similarly, the node 115 terminates the signal number Sig1 received from the node 112. Because of this, the client signal to be transmitted from the node 112 to the node 111 at the spare ODU is terminated at the node 115, resulting in a misconnect (arrow A102 in FIG. 15).